In an arrangement in which a plurality of nodes are connected to a common bus, some kind of scheme is needed to select which node will be able to use the bus at any given time. One scheme which has been used in prior systems is to assign a fixed priority to each of the nodes connected to the bus so that if two of the nodes desire to use the bus at the same time, the node having the higher priority will win every time. The advantage of using a fixed priority is that the nodes which need to use the bus the most have the most opportunity to do so. The scheme has an inherent disadvantage in that the higher priority nodes can effectively monopolize the use of the bus so that lower priority nodes may never have the opportunity to use the bus.
Another scheme which has been used in the past is a round robin arbitration. Each node is sequentially enabled in round robin fashion so that there is an equal sharing of the bus between the nodes. Although this provides equal access to all of the nodes, the scheme can be inefficient since nodes which need to use the bus more often than other nodes have to wait for the round robin to complete before being able to use the bus again. However, the nodes which use the bus less frequently are less likely to actually use the bus during their turn, so there is often a waste of the time reserved for nodes which use the bus less frequently.